Blasting events generally cause damage and stresses in the ground and/or in man-made structures, that extend beyond the immediate area or region of the event, e.g., outside the perimeter of a mining excavation. For example, blast vibration in open pit mines is a contributor to highwall damage, and failure of such walls can cause loss of reserves and interruptions to mine production. Poor stability and failure can be expensive, requiring recovery of ore by clean-up operations, modification of a mine plan, construction of new ramps, installation of extra ground support, and lost production during the remediation period. In underground mines, blast vibration can cause instability in large open stopes, caving operations and mine development tunnels.
It can therefore be desirable to measure vibrations in an attempt to determine whether an amount of vibration due to one or more events, e.g., blasting events, may lead to instability.
In existing systems and processes, vibrations in structures have typically been analysed based on measured motions of the particles in the structures, and subsequent determination of the peaks in velocity, acceleration or displacement of the particles, referred to as the peak particle velocity (PPV), the peak particle acceleration (PPA) or the peak particle displacement (PPD). Once determined, the PPV, PPA and/or PPD can be compared to preselected or predefined acceptable upper limits on peak velocity, acceleration or displacement to determine a likelihood of possible instability or damage in parts of the structure. These acceptable upper vibration velocity, acceleration and displacement limits are, however, difficult to determine accurately, appropriately and consistently, e.g., for each particular application or operation, and selecting incorrect PPV, PPA or PPD limits can be problematic. On the one hand, if the blast vibration limit (i.e., the preselected PPV, PPA or PPD limit) is too low, the cost of the operation can be unnecessarily high. On the other hand, if the vibration limit is too high, it can result in unsafe practices and undesirable damage to the natural structure (e.g., the rock or soil) and/or to other nearby structures including man-made structures.
Some existing systems and processes may use particle velocity to estimate deformation in a structure, such as strain in rock; however, the inventors have realised that these systems typically assume that the strain is linearly proportional to the particle velocity at a point, and thus may not be sufficient for estimating stress and strain in the vicinity (e.g., within a few vibration wavelengths, which can be referred to as being in the near field) of a vibration source (e.g., a blast point or focus) and/or due to relatively rapid transient events, e.g., blasting events.
The motions of the particles in the structure or structures can be related to each other through various forms of mechanical waves such as particle displacement, particle velocity, particle acceleration vibration waves, stress waves or strain waves propagating in the structure(s). The mechanical waves can also be referred to as motion waves.
Some existing systems and processes may use simple wave models (e.g., based on spherical, cylindrical and/or plane waves) to estimate propagation of the motion waves in structures (e.g., for plane waves it is based on a one-dimensional (1D) relationship between particle velocity and strain at each point); however, these simple models may be insufficient for determining the stress and strain caused by a relatively rapid event, such as a blast, and/or in the near field of the vibration source. Furthermore, for blasting, simple wave models may be unsuitable if blast holes are loaded with various different explosive types, if the explosives are delayed individually (i.e., they have different timing), or if the geology of the structure contains layers of different materials and/or other non-uniform structural or material zones. Non-uniformity in the structure (e.g., the blast medium) can cause propagating waves to be reflected, refracted, and transformed between modes, e.g., between waves in the medium and surface waves on the surface of the medium, and between waves of different types. The different types of waves include primary (P) waves (also referred to as compressional waves) whose particle motion is in the direction of propagation, secondary (S) waves (also referred to as shear waves) whose particle motion is perpendicular to the direction of propagation and which can have different polarisations, and surface waves such as Rayleigh (R) waves whose particle motion is elliptical, Love (L) waves and other waves that travel as guided waves.
It is desired to address or ameliorate one or more disadvantages or limitations associated with the prior art, or to at least provide a useful alternative.